Re: [PATCH v9 3/8] KVM: Add KVM_EXIT_MEMORY_FAULT exit

Alex Bennée
2022-11-16T19:07:15Z

Chao Peng <chao.p.peng@linux.intel.com> writes:

> On Tue, Nov 15, 2022 at 04:56:12PM +0000, Alex Bennée wrote:
>>
>> Chao Peng <chao.p.peng@linux.intel.com> writes:
>>
>> > This new KVM exit allows userspace to handle memory-related errors. It
>> > indicates an error happens in KVM at guest memory range [gpa, gpa+size).
>> > The flags includes additional information for userspace to handle the
>> > error. Currently bit 0 is defined as 'private memory' where '1'
>> > indicates error happens due to private memory access and '0' indicates
>> > error happens due to shared memory access.
>> >
>> > When private memory is enabled, this new exit will be used for KVM to
>> > exit to userspace for shared <-> private memory conversion in memory
>> > encryption usage. In such usage, typically there are two kind of memory
>> > conversions:
>> > - explicit conversion: happens when guest explicitly calls into KVM
>> > to map a range (as private or shared), KVM then exits to userspace
>> > to perform the map/unmap operations.
>> > - implicit conversion: happens in KVM page fault handler where KVM
>> > exits to userspace for an implicit conversion when the page is in a
>> > different state than requested (private or shared).
>> >
>> > Suggested-by: Sean Christopherson <seanjc@google.com>
>> > Co-developed-by: Yu Zhang <yu.c.zhang@linux.intel.com>
>> > Signed-off-by: Yu Zhang <yu.c.zhang@linux.intel.com>
>> > Signed-off-by: Chao Peng <chao.p.peng@linux.intel.com>
>> > ---
>> > Documentation/virt/kvm/api.rst | 23 +++++++++++++++++++++++
>> > include/uapi/linux/kvm.h | 9 +++++++++
>> > 2 files changed, 32 insertions(+)
>> >
>> > diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
>> > index f3fa75649a78..975688912b8c 100644
>> > --- a/Documentation/virt/kvm/api.rst
>> > +++ b/Documentation/virt/kvm/api.rst
>> > @@ -6537,6 +6537,29 @@ array field represents return values. The userspace should update the return
>> > values of SBI call before resuming the VCPU. For more details on RISC-V SBI
>> > spec refer, https://github.com/riscv/riscv-sbi-doc.
>> >
>> > +::
>> > +
>> > + /* KVM_EXIT_MEMORY_FAULT */
>> > + struct {
>> > + #define KVM_MEMORY_EXIT_FLAG_PRIVATE (1 << 0)
>> > + __u32 flags;
>> > + __u32 padding;
>> > + __u64 gpa;
>> > + __u64 size;
>> > + } memory;
>> > +
>> > +If exit reason is KVM_EXIT_MEMORY_FAULT then it indicates that the VCPU has
>> > +encountered a memory error which is not handled by KVM kernel module and
>> > +userspace may choose to handle it. The 'flags' field indicates the memory
>> > +properties of the exit.
>> > +
>> > + - KVM_MEMORY_EXIT_FLAG_PRIVATE - indicates the memory error is caused by
>> > + private memory access when the bit is set. Otherwise the memory error is
>> > + caused by shared memory access when the bit is clear.
>>
>> What does a shared memory access failure entail?
>
> In the context of confidential computing usages, guest can issue a
> shared memory access while the memory is actually private from the host
> point of view. This exit with bit 0 cleared gives userspace a chance to
> convert the private memory to shared memory on host.

I think this should be explicit rather than implied by the absence of
another flag. Sean suggested you might want flags for RWX failures so
maybe something like:

KVM_MEMORY_EXIT_SHARED_FLAG_READ (1 << 0)
KVM_MEMORY_EXIT_SHARED_FLAG_WRITE (1 << 1)
KVM_MEMORY_EXIT_SHARED_FLAG_EXECUTE (1 << 2)
KVM_MEMORY_EXIT_FLAG_PRIVATE (1 << 3)

which would allow you to signal the various failure modes of the shared
region, or that you had accessed private memory.

>
>>
>> If you envision any other failure modes it might be worth making it
>> explicit with additional flags.
>
> Sean mentioned some more usages[1][]2] other than the memory conversion
> for confidential usage. But I would leave those flags being added in the
> future after those usages being well discussed.
>
> [1] https://lkml.kernel.org/r/20200617230052.GB27751@linux.intel.com
> [2] https://lore.kernel.org/all/YKxJLcg%2FWomPE422@google.com
>
>> I also wonder if a bitmask makes sense if
>> there can only be one reason for a failure? Maybe all that is needed is
>> a reason enum?
>
> Tough we only have one reason right now but we still want to leave room
> for future extension. Enum can express a single value at once well but
> bitmask makes it possible to express multiple orthogonal flags.

I agree if multiple orthogonal failures can occur at once a bitmask is
the right choice.

>
> Chao
>>
>> > +
>> > +'gpa' and 'size' indicate the memory range the error occurs at. The userspace
>> > +may handle the error and return to KVM to retry the previous memory access.
>> > +
>> > ::
>> >
>> > /* KVM_EXIT_NOTIFY */
>> > diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
>> > index f1ae45c10c94..fa60b032a405 100644
>> > --- a/include/uapi/linux/kvm.h
>> > +++ b/include/uapi/linux/kvm.h
>> > @@ -300,6 +300,7 @@ struct kvm_xen_exit {
>> > #define KVM_EXIT_RISCV_SBI 35
>> > #define KVM_EXIT_RISCV_CSR 36
>> > #define KVM_EXIT_NOTIFY 37
>> > +#define KVM_EXIT_MEMORY_FAULT 38
>> >
>> > /* For KVM_EXIT_INTERNAL_ERROR */
>> > /* Emulate instruction failed. */
>> > @@ -538,6 +539,14 @@ struct kvm_run {
>> > #define KVM_NOTIFY_CONTEXT_INVALID (1 << 0)
>> > __u32 flags;
>> > } notify;
>> > + /* KVM_EXIT_MEMORY_FAULT */
>> > + struct {
>> > +#define KVM_MEMORY_EXIT_FLAG_PRIVATE (1 << 0)
>> > + __u32 flags;
>> > + __u32 padding;
>> > + __u64 gpa;
>> > + __u64 size;
>> > + } memory;
>> > /* Fix the size of the union. */
>> > char padding[256];
>> > };
>>
>>
>> --
>> Alex Bennée

--
Alex Bennée